PvP mounts are earned through Player-vs-Player gameplay. The Honor mounts can be bought with Honor Points;
the Halaa and Wintergrasp mounts require tokens that you can earn during battles in those locations;
and the Arena mounts can only be earned by being exceptionally good at 3-person or 5-person Arena team competitions.
Click on a mount to learn more about it.
Honor - Alliance
Honor - Horde
Rated Battleground Achievement
Arena mounts are rewarded to top-ranking Arena teams at the end of each Arena season. Since each mount is given out for only one Arena season, Arena mounts are effectively pre-retired -- by the time they are in the game, you can't get one. You can find more retired mounts on the Retired Mounts